1. Field of the Invention
The present invention relates generally to an automatic focusing system for cameras. More specifically, the invention relates to an automatic focusing system which can accurately measure the distance between the camera and the object to be focused upon under all environmental conditions and for all object brightnesses. Further particularly, the invention relates to an automatic focusing system applicable for still cameras, such as electronic cameras, for example.
2. Description of the Background Art
In recent years, various automatic focusing systems for cameras have been developed for automatically measuring distance between the cameras and objects to be focused upon and operating focusing lens at positions corresponding to the measured distance for focusing at the object. Such automatic focusing systems are useful in various cameras, such as electronic still cameras and so forth, for eliminating the need for operators of the cameras to focus them.
As is well known, basically two different types of automatic focusing systems have been developed. One of the systems is known as an "active type automatic focusing system" which transmits an infrared light beam or ultra sonic wave toward the object to be focused upon and receives the reflected beam or wave for measuring the distance from the camera to the object by measuring elapsed time from transmission to receipt. The other type of system is known as a "passive type automatic focusing systems" which delects environmental light reflected by the object for deriving the focusing lens position based thereon. As may easily be appreciated, the active type automatic focusing system requires an infrared light beam generating means or ultra an sonic wave generating means for transmitting the light beam or ultra sonic wave toward the object. On the other hand, since the passive type automatic focusing systems are not required to facilitate any light beam or ultra sonic wave generator, the construction of these systems is much simpler than that of the active type automatic focusing system. Also, the passive type automatic focusing systems are known to require less electric power than the active type systems. Among various passive type automatic focusing systems, so-called TTL (through the lens) systems are known to be useful because they are subject to no parallax in focusing.
On the other hand, the passive type automatic focusing system tends to have difficulty in focusing upon relatively dark objects, since light intensity received from dark objects tends to be insufficient for performing automatic focusing operations. Therefore, when the environmental condition in picking up an image of the object or taking a picture requires strobe light or flash light, it is practically impossible to perform automatic focusing operations by means of passive type automatic focusing systems, before the strobe light is emitted. Therefore, the passive type automatic focusing system is indeed useless in focusing in dark environmental conditions or for focusing upon dark objects.